This invention relates to viewing objects in optically opaque media and particularly to a method and apparatus for insonifying the subject and transforming reflected or transmitted ultrasonic waves directly into an optical image--an optical image capable of being viewed and reproduced by photographic means.
While ultrasonic viewing or exploratory systems have a wide range of applications and the methods and system described here may be employed in the full range of applications, the most exacting application, and the use for which the system is particularly suited, is that of imaging living organisms, e.g., organs such as the heart in a living human body. In view of the special applicability to medical uses, the description is cast in terms of this most exacting of applications but use of the methods and apparatus is not so limited.
The contemplated major application leads to a consideration of its competitors in terms of imaging internal organs in living organisms. Techniques which do not provide an image are not considered true competitors. Radiography does provide an image in some cases and has demonstrated that the presentation of diagnostic information in the form of an image bearing a one-to-one correspondence to the anatomy involved is a most powerful perceptual aid.
Although radiographic techniques are basically noninvasive, in many cases catheterization or injection of contrast media is required for effective visualization. Always present are the risks of exposure to ionizing radiation, both to the patient and to the radiologist. Additionally, there are many situations in which x-rays fail to provide adequate visualization of tissues and organs. In such cases, the diagnostician must depend more heavily on other established means-- physical examination, chemical tests, measurement of electrical potentials, etc. For example, the normal thyroid gland is not demonstrable on x-ray studies; the abnormal gland is demonstrated only when it contains calcified areas or by its effect on neighboring structures. Present methods of diagnosis cannot adequately distinguish malignant from nonmalignant thyroid growths.
Radiographic techniques have proven deficient in visualization of the brain, some bone structure and detecting tumors in regions such as the breast. Specifically, radiographic visualization of the brain is carried out by visualizing its ventricles and basal cisterns with the injection of opaque media or gas, with the risks that attend the procedure. Arterial dye injections are required to visualize the heart and circulatory system, which is otherwise demonstrable only when calcification is present. Many details of the internal structures of abdominal organs are not apparent in simple radiographic examination; the pancreas, for example, cannot be distinguished from the surrounding soft tissues. Again, contrast media, either for absorption by the organ's tissue or passage through its circulatory system, are required in order to delineate the organ.
Radiographic visualization of the bones in the extremities is satisfactory for most purposes. However, visualization of the soft tissue, cartilage and joint cavities of the extremities cannot be adequately performed with any x-ray technique.
Mammography--a radiographic technique for detecting tumors of the breast--has found increased acceptance. However, in order to detect the minute calcifications that occur in malignant tumors, very high resolution film is used without intensifying screens. This film is relatively insensitive and requires long exposure. Thus a considerable dose of the readily absorbed soft x-rays is delivered to the patient. The inadequate interaction of x-rays with soft tissue and the complications associated with the use of contrast media significantly restrict the application of radiography. A further disadvantage of x-rays is that they interact only with tissue volume and are insensitive to interfaces between tissues. There is a clear need for new procedures by which internal organs can be visualized without using ionizing radiation and without introducing foreign material into the body.
The ability of ultrasonics to differentiate tissues on the basis of their elastic properties, the lack of toxic effects at energy levels required for diagnostic use and the fact that there is no requirement for invasive techniques with their attendant disadvantages and dangers make ultrasonic visualization a particularly attractive and effective diagnostic tool. A variety of ultrasonic techniques have been demonstrated. A discussion of the various methods and their capabilities is found in INTERNATIONAL JOURNAL OF NONDESTRUCTIVE TESTING, vol. 1 (1969), pp. 1-27, "Methods of Acoustic Visualization," by Philip S. Green, one of the present inventors. Although the referenced article gives a rather complete discussion, and its subject matter is specifically incorporated herein by reference, a discussion of the techniques is given here to establish the background of the present invention.
The ultrasonic technique first introduced and now most widely used in diagnostic medicine is the sonar-like method wherein an ultrasonic transducer in contact with the patient's skin launches ultrasonic pulses into the tissue and subsequently detects their reflections from tissue interfaces. The method is labeled the "A-scan" technique. Since the round trip travel time of a reflected pulse is proportional to the distance from the transmitter to the reflecting layer, the presentation of these wave trains on an oscilloscope permits the operator to directly measure these distances. This method has found wide acceptance as an indicator of midline displacement caused by tumors of the brain.
Aside from doppler methods, in which tissue or fluid motion is detected by means of the frequency shift it imparts to reflected ultrasonic waves, the only other method of diagnostic ultrasound to achieve a significant level of use is the so called "B-scan." As with the A-scan, the B-scan method of ultrasonic diagnosis employs a narrow beam transducer to project a short ultrasonic pulse into the tissue and to detect the reflected pulses. In B-scan, however, a two dimensional image is produced by moving the transducer slowly past the area of interest and recording the reflected pulse trains at closely spaced intervals. A cathode ray tube display is used in which one of the orthogonal deflection voltages is proportional to the transducer position and the other to the time elapsed since the last pulse was transmitted. The reflected ultrasonic pulses intensity-modulate the display. The resulting image is of a section of the body or organ that lies in the plane of the propagating rays.
The success of B-scan imaging has been most striking in obstetrics. Cross sections of the fetal head, thorax and limbs can be displayed with enough clarity to determine fetal size and position. B-scan is also useful in the differentiation of abnormal and normal pregnancy and for localization of the placenta.
Application of B-scan to the abdominal organs has met with more modest success. It is frequently possible to detect the presence of an abnormal state in the liver. However, except for the differentiation of cystic and solid masses, it is rarely possible to identify the abnormality. It is generally recognized that practical application of ultrasound for diagnosis of liver diseases is hampered by both inadequaces in equipment and examining techniques. The B-scan technique has been used successfully to measure the volumes of both liver and spleen.
B-scan has been employed for the measurement of cross sectional area in many organs, including the kidney and bladder. Again, although echoes are frequently observed from the interior of diseased kidneys, it is seldom possible to correlate these echo patterns with any specific pathological change.
In a B-scan study of the thyroid, four basic types of echo patterns have been distinguished and, in some cases, correlated with specific pathologies. It is found that cystic nodules rarely produce ultrasonic echoes. The results with solid adenomas are much less consistent; in the case of colloid adenoma, the ultrasonic "tomograms" are often echo free in the thyroid area. Almost all of the carcinomas investigated have been papillary, and their tomograms typically contained many echoes, although they were divided between two of the designated patterns. Nonneoplastic lesions--for the most part diffuse thyroiditis and Graves' disease--are not well characterized by the tomograms.
Detection of neoplasms with the B-scan technique has been reported. As with most other organs, differentiation can be made between cystic and solid masses. Although this early work gave rise to hope that distinctive echo patterns were associated with malignant and benign solid masses, it is the opinion of many investigators that this differentiation cannot be made on the basis of B-scan images. Visualization of the breast in this manner is further complicated by the extent and variability of sonic scattering by the internal structure of the normal breast.
Application of B-scan to visualization of the brain is hampered by the considerable reflection and refraction of sound that occurs at the skull. Relatively little work had been done in this area until recently, when in several investigations it was shown that some structures within the brain can be delineated if care is taken to discriminate against secondary reflections from the skull.
Image-like visualization of the heart is achieved by holding the transducer still while recording the output on a moving strip chart recorder. In this manner, motion of the heart wall and mitral valve results in wavy lines in the recording. The amplitude of these oscillations is a direct measure of valve and wall displacement, while valve and wall thickness can be deduced from the thickness of the lines. The heart structures are not actually resolved with this technique.
Although many other organs have been visulaized visualized B-scan, the only one of these to have received intensive investigation is the eye. Tumors of the eye and retinal detachments are readily displayed, as is some of the normal structure of the eye.
B-scan has established the potential effectiveness of ultrasonic visualization in medical diagnosis and is particularly effective for determining cross sectional shapes and areas of internal organs. However, B-scan has several significant inherent deficiencies of which the following are noteworthy:
Poor lateral resolution: In order to ensure that the ultrasonic beam is well collimated, it is necessary to use a transducer that is many wavelengths in diameter. The lateral resolution is limited to the transducer diameter--typically 0.5 to 2 cm. If a highly focused transducer is used, a sharp focus will prevail at the focal depth, while at other depths the image will be grossly out of focus. PA0 Difficulty with specular reflection: Often tissue interfaces have relatively little irregularity on the scale of the ultrasonic wavelength (usually 0.3 to 1.0 mm) and thus reflect waves in a specular or mirror-like manner. It has become general practice to sector scan the transducer rapidly as it transverses the area of interest so that reflections from specular surfaces are less likely to be undetected. However, this provision effects only a partial solution to the specularity problem while introducing a great deal more complexity. PA0 Insensitivity to local variations in acoustic absorption: The differences in acoustic absorption between normal and abnormal tissues may in some cases be a more sensitive indicator than differences in acoustic impedance. The pulse echo methods are essentially insensitive to local variations in absorptivity. PA0 Long "build-up" time of image: Because of the finite time required for a transmitted pulse to return from the most distant portion of the organ and because of mechanical complexity, the time required to build up a single cross sectional image may be several minutes. Not only must the patient be kept immobile during this time to prevent image smear, but the diagnostician must forego the considerable advantage of seeing an image in "real time." Furthermore, five to ten scans are usually required to completely characterize an organ. PA0 Low dynamic range: Although tissue reverberations span a wide range of amplitudes, the storage tube display typically used in B-scan imaging presents only a two level (black-white) approximation of the image.
In an effort to overcome the problems of the A-scan and B-scan techniques for diagnostic visualization, two other techniques of ultrasonic visualization have been investigated, viz., the lens/converter method and acoustic holography. An important distinction between the results obtained with the B-scan method and those obtained with both lens/converter and acoustic holography methods is found in the type of image produced.
Whereas the B-scan image is of a section of the object lying in the plane of the propagating rays, the image produced with lens/converter and acoustic holography systems is of a plane (or planes) normal to the propagation direction. The latter's similarity to our everyday experience of visualization leads to greater ease of interpretation. Furthermore, while B-scan images are formed with reflected energy only, images may be produced with either transmitted or reflected energy by using the lens/converter method or acoustic holography method, in analogy to the two most common modes of optical microscopy. A theoretical foundation for the lens/converter mode of acoustic image formation is found in P. S. Green, J. L. S. Bellin and G. C. Knollman, "Acoustic Imaging in a Turbid Underwater Environment", J. ACOUST. SOC. AM., vol. 44 (1968), pp. 1719-1730.
Considering the lens/converter method, the use of lenses or focusing reflectors to concentrate ultrasonic energy is well known. A close analogy exists between the reflection and refraction of acoustic and optical wavefronts at boundaries separating regions of different refractive index, and sonic lenses and reflectors are designed in accordance with the same procedures used in optics. Indeed, the analogy between acoustics and optics extends to all scalar propagation phenomena. Then, as we might expect, there exists for an acoustic lens or focusing reflector an image plane/object plane relationship identical to that found in optics. Specifically, a spatial pattern of acoustic pressure in a plane in front of an acoustic lens (and propagating toward it) will induce in the conjugate plane of the lens a diffraction and aberration limited replica of itself. As in optics, this replica may be a virtual image for certain parameter values. A real image of an insonified object is formed if the object is placed beyond the focal plane of a convergent lens. An acoustic image converter placed in the corresponding image plane (as determined by the lens law) will detect a focused acoustic image of the object. The method and apparatus for ultrasonic imaging which constitute the present invention are a lens/converter system.
As pointed out above, the acoustic holography method of imaging can produce images of the same form as those resulting with the lens/converter system. In the case of holography, the acoustic lens may be omitted, and the wavefield scattered by the object is sampled directly by the converter. An optical transparency representing this wavefield is then formed by one of several conversion methods. Illumination of this transparency by a laser produces both a real and virtual image of the insonified object.
In recent years the holographic method of acoustic visualization has received considerable attention. Various methods of implementation have been demonstrated. The holographic approach, however, has been found to have several serious drawbacks. The formation of a holographic reconstruction requires first that the ultrasonic wavefield be recorded on film and that the film be developed; this is a time consuming operation producing a single image and is incompatible with the desired real time viewing capability. Although real time holography is possible with a liquid surface relief conversion method, this method is considered too insensitive for diagnostic use, a judgment based on laboratory experience with the liquid surface relief conversion process as used to make nonholographic, real time, in vitro images of various organs. Although work is now in progress to develop real time, electronically addressable transparencies for acoustic holography, results to date have been discouraging. A further disadvantage of ultrasonic holography is that coherent ultrasonic waves must be used for object insonification. It has been our experience, substantiated by theoretical studies, that broadband sound produces images of greater fidelity. Although in optical holography the reconstructed image is seen in three dimensions, this advantage is not realized in the acoustic case, owing to the inherent distortion caused by the large disparity between the wavelengths of ultrasound and light.